Corrosion-Involved Degradation of Ultra High Strength Steel

Abstract

In this work, the sulfide corrosion cracking susceptibility of a novel ultrafine grain high strength steel were studied under different concentrations of H2S and pH. The critical sulfide stress cracking factor (KISSC ) was applied to characterize the sulfide corrosion cracking susceptibility based on fracture mechanics. Increasing either H2S concentration or pH increases KISSC . KISSC displays an exponential decrease with H2S activity. Fracture surface morphology shows a clear boundary of ductile and brittle fracture region. The density of secondary cracks in ductile region and brittle fracture area increases with H2S content. Hydrogen permeation testing was performed to obtain the subsurface concentration of atomic hydrogen (C0 ). The relationship between KISSC and C0 was correlated to study the influence of hydrogen at crack tip on KISSC . Similar to other high strength steels, the ultrafine grain high strength steel was found to be susceptible to sulfide stress cracking and show a comparable to commonly used C110 steels. However, the ultrafine grain high strength steel has a much higher strength than C110 steels. To evaluate the overall performance, the corrosion behavior of the ultrafine grain high strength steel was monitored in acidic H2S environment by electrochemical measurements. A typical active dissolution occurred and the corrosion resistance was found to be higher than the commonly used X100 carbon steel in the literature. From the aspects of sulfide corrosion cracking susceptibility and corrosion performance, the studied ultrafine grain high strength steel is promising to be applied in oil and gas field. 18Ni maraging steel, as a high strength steel, shows a similar yield stress to ultrafine grain high strength steel and better mechanical properties after aging treatment. The second part of this proposal is to study the materials degradation of maraging steel prepared by selective laser melting (SLM) technology from the aspect of corrosion performance. The peak-aged condition was considered to study the effects of heat treatment on corrosion behavior of maraging steel with the superior mechanical properties. The SLM specimens showed fine microstructures with homogeneous cellular and column dendrites, which resulted in relative uniform corrosion. The aging treatment accelerated corrosion rate with a thick layer of corrosion products covered on the top of metal surface. Electrochemical impedance spectroscopy (EIS) shows a better corrosion resistance of aged samples in a short term but a severer corrosion for a long time immersion. The corrosion rate of aged samples was tested to be higher than non-aged samples by using direct current method. Compared to either other traditional manufacturing methods (like welding) or additive manufacturing technique (like laser powder bed fusion), SLM manufactured maraging steel in this work has a much lower corrosion rate.